Monolithic catalyst system for the photolysis of water

ABSTRACT

A monolithic catalyst system for the cleavage of water into hydrogen and oxygen comprises a first photoactive material capable by itself or together with an auxiliary material and/or an auxiliary catalyst when irradiated with light having a wavelength≧420 nm of generating oxygen and protons from water, and a second photoactive material capable by itself or together with an auxiliary material and/or an auxiliary catalyst when irradiated with light having a wavelength≧420 nm of reducing protons in water to hydrogen. The first and second photoactive materials are in electrical contact via an electron-conducting material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Stage of InternationalApplication No. PCT/EP2008/009229, filed Oct. 31, 2008, which claims thebenefit under 35 U.S.C. 119(e) of U.S. Provisional Application No.61/130,701, filed Jun. 2, 2008.

BACKGROUND OF THE INVENTION

The invention relates to a catalyst system for the cleavage of waterinto hydrogen and oxygen with the aid of light and to a method ofproducing hydrogen and oxygen using the catalyst system.

PRIOR ART

Hydrogen is generally believed to become the material energy carrier ofthe future and thus there is a major interest in the environmentallyfriendly production of hydrogen without the concomitant production ofcarbon dioxide and without the use of conventional electrolysis whichusually is expensive and often environmentally unfriendly.

In U.S. Pat. No. 6,936,143 B1 Graetzel et al. disclosed a tandem cell orphotoelectron-chemical system for the cleavage of water to hydrogen andoxygen by visible light, both cells being connected electrically. Thiselectrical connection involves an organic redox electrolyte for thetransport of electrons from the photoanode, e.g. WO₃ or Fe₂O₃ to thephotocathode, a dye sensitized mesoporous TiO₂ film. Although nothing isdisclosed in this patent about the organic redox electrolyte, it isclear the very term itself involves an electron transport through ionicconduction, since electrolytes always transport charge thorough ions.

SUMMARY OF THE INVENTION

The present invention provides a monolithic catalyst system for thecleavage of water into hydrogen and oxygen with the aid of light,comprising a first photoactive material capable by itself or togetherwith one or more of an auxiliary material and an auxiliary catalyst whenirradiated with light having a wavelength≧420 nm of generating oxygenand protons from water, and a second photoactive material capable byitself or together one or more of an auxiliary material and an auxiliarycatalyst when irradiated with light having a wavelength≧420 nm ofreducing protons in water to hydrogen, the first photoactive materialand second photoactive material being in electrical contact,particularly in direct electrical contact, via one or moreelectron-conducting materials.

Also provided is a method of generating oxygen and hydrogen from waterwith the aid of light and a catalyst system which is characterized inthat a catalyst system in accordance with the invention is brought intocontact with water or an aqueous fluid or solution at a first locationcomprising a first photoactive material or one or more of an auxiliarymaterial and an auxiliary catalyst associated therewith or both andwhich is brought into contact with water or an aqueous fluid or solutionat a second location comprising the second photoactive material or oneor more of an auxiliary material and an auxiliary catalyst associatedtherewith or both and is then irradiated with light, the water oraqueous fluid or solution in contact with the first location and thewater or aqueous fluid or solution in contact with the second locationbeing in contact with each other such that protons can migrate from thefirst location to the second location.

Advantageous embodiments of the invention are recited in the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the so-called the Z scheme of photosynthesis orphotolysis of water in plants or bacteria as used in the catalyst systemin accordance with the invention.

FIG. 2 depicts a diagrammatic cross-section through an example of acatalyst system in accordance with the invention.

FIG. 3 depicts the UV/Vis spectrum oftris[4-(11-mercaptoundecyl)-4′-methyl-2,2′-bipyridin]ruthenium(II)-bis(hexafluorophosphate).

FIG. 4 depicts the UV/Vis spectrum of[(2,2′-bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-[(tetrapyridophenazine)-palladium(II)-dichloro]-bis(hexafluorophosphate)

FIG. 5 depicts the UV/Vis spectrum of Mn₄O₄(phenyl₂PO₂)

PRINCIPLE OF GENERATING HYDROGEN AND OXYGEN ACCORDING TO THE SO-CALLEDZ-SCHEME

The catalyst system of the present invention uses four photons for thecleavage of water into ½O₂ and H₂. The various partial steps and howthey relate by their energy levels are depicted diagrammatically in FIG.1.

Required at the oxidizing side of the catalyst system (also termed firstphotoactive material hereinafter) are 2 photons for the followingreactionH₂O+2 photons→½O₂+2H⁺+2e ⁻(E_(O2/H2O(pH7))=+0.82 V).

This is the reaction that takes place in plants/bacteria in theso-called photosystem 2.

Required at the reduction side of the catalyst system (also termedsecond photoactive material hereinafter) are 2 photons for the followingreaction2H⁺+2e ⁻+2 photons→H₂(E_(H+/H2(pH7))=−0.41 V).

This is the reaction that may take place in some bacteria in thephotosystem 1 in conjunction with the enzyme hydrogenase that generateshydrogen.

The net result of this reaction is:H₂O+4 Photonen→H₂+½O₂(E_(pH7)=1.23 V).

What is involved is thus a process in which 2 photons (2 hν) are neededso that 1 e⁻ is removed from the oxygen in water and transferred to a H⁺ion (2 hν→1 e⁻). This reaction is also termed Z scheme reactionaccording to photosynthesis in plants and bacteria.

The terms “hydrogen”, “protons”, “H⁺”, “H⁺ ions” etc. in conjunctionwith the present invention are also intended to include the terms“deuterium”, “deuterium ions”, “D⁺”, “D⁺ ions” etc. Likewise the term“H₂” is also intended to include “HD” and “D₂”. However, the term “D₂”does not include “HD” and “H₂”.

The electrons set free at the oxidation side of the catalyst system (inthe terms of electrochemistry: the anode) in accordance with theinvention are conducted directly to the reduction side of the catalystsystem (in the terms of electrochemistry: the cathode) via one or moreelectron-conducting materials. Ion conductors, fluid redox electrolytesand solid electrolytes are not included in the term “electron-conductingmaterial”. Electron conduction through junctions such as a p-n junctionis not considered to involve an electron conducting material between afirst and a second photoactive material in the sense of the presentinvention.

The invention imitates the Z scheme of plants and bacteria also inregards to the direct electron conduction between the two photoactivematerials. Thus, it truly artificially mimics photosynthesis in plantsand bacteria or, more precisely, the photolysis of water in somebacteria, i.e. can truly be considered to achieve artificialphotosynthesis.

Of course, no external voltage has to be applied to the system in orderto function.

A monolithic catalyst system in this application is understood to be asystem which is compact and has no structures such as macroscopic wires,conductors or electrodes extending from the system and not compactlyintegrated therein, e.g. no electrodes which are connected to the systemvia a conductive wire, band or sheet or the like. Such a monolithicsystem may take the form of a plate, a film or also a tube. “Monolithic”is not intended to mean that the system is necessarily fabricated as asingle piece.

A “photoactive material” is understood in this patent application to bea material which together with a further photoactive material shows aredox potential scheme corresponding to the Z scheme of thephotosynthesis/photolysis, the total potential difference of which issufficient to permit cleavage water into hydrogen and oxygen when thephotoactive materials are irradiated with light having a wavelength≧420nm, preferably ≧430 nm, more preferably ≧440 nm and particularly ≧450nm. Furthermore, preferably one or both of the photocatalysts should notexclusively absorb electromagnetic radiation at wavelengths ≧700 nm.

As evident from the Z scheme (see FIG. 1) the redox potentials of thefirst and second photoactive material comprise the following redoxpotentials and redox potential relationships:

-   1. The redox potential of the ionized state of the first photoactive    material and the redox potential of the positively charged valence    band of the first photoactive material, respectively, is more    positive than +0.82 V.-   2. The redox potential of the excited state of the second    photoactive material and the redox potential of the conduction band    of the second photoactive material, respectively is more negative    than −0.41 V.-   3. The redox potential of the excited state of the first photoactive    material and the redox potential of the conduction band of the first    photoactive material, respectively, is more negative than the redox    potential of the ionized state of the second photoactive material    and the positively charged valence band of the second photoactive    material, respectively.

The redox potential of the non-excited state of the first photoactivematerial and of the valence band of the first photoactive material,respectively, is, as a rule more positive than the redox potential ofthe non-excited state of the second photoactive material and of thevalence band of the second photoactive material, respectively. Since thecatalyst system is required to work with visible light having awavelength ≧420 nm, the excited states and the conduction bands,respectively, of the photoactive materials must permit being generatedor occupied with the aid of light of such a wavelength.

The term “a first photoactive material” (which in the terms ofelectrochemistry is the anode of the photocatalyst system or forms partthereof) and “a second photoactive material” (which in the terms ofelectrochemistry is the cathode of the photocatalyst system or formspart thereof) is understood to also mean a plurality (or a mixture) offirst photoactive materials and second photoactive materials,respectively.

A variety of materials, both in the form of non-molecular solids as wellas molecular and polymer compounds, is known which may serve as thefirst photoactive (oxidation-promoting) material and work in lighthaving a wavelength≧420 nm. The first photoactive (oxidation-promoting)material may, however without being limited thereto, comprise anoptionally doped oxide- and/or sulfide-containing material, inparticular RuS₂, complexes or clusters containing a noble metal or antransition metal, and photoactive polymeric materials. For example andwithout limitation, use may be made of RuS₂ which may be doped, WO₃,which may comprise a noble metal, an iron oxide, which may be doped withforeign atoms, TiO₂ doped with Sb/M (M=Cr, Ni and/or Cu), a Mn₄ cagecomplex, a Ru₄ cluster complex, a Ru³⁺ complex and the specificphotoactive materials described in examples 1, 2 and 3 of the presentapplication.

It is often the case that the individual mechanistic steps resulting inrelease of electrons from the oxygen of water are not known precisely.But in any case an electronically excite state is created with the aidof a photon having a wavelength≧420 nm, the excited electron whenspatially separated from the first photoactive material leaves behindtherein an oxidation state elevated by 1 or a hole which is filled by anelectron of the oxygen of the water so that ultimately O₂ and protonsare generated, it often being the case that also 2, 3 or 4 (additional)positive charges or holes are generated simultaneously.

To facilitate development of oxygen the first photoactive material maybe associated with an auxiliary material and/or catalyst which itself isnot a photoactive material as defined above, it instead promoting oxygendevelopment without being able to develop oxygen by itself underirradiation. Such auxiliary materials and/or catalysts are withoutlimitation e.g. RuO₂, certain noble metals, such as palladium orplatinum, or a compound formed in situ from cobalt metal and a phosphatein water.

In use of the catalyst system either the first photoactive material orthe auxiliary material and/or catalyst, where existing, or both are incontact with water.

For the second photoactive (reduction promoting) material too, a wealthof materials exists, both in the form of non-molecular solids as well assolid molecular and polymer compounds which absorb in the wavelengthrange ≧420 nm, all of which may be used. For example, such a materialmay be one of the many ruthenium(II) complexes most often complexed withnitrogen-containing ligands, of which it is known that they when excitedwith light of a wavelength≧420 nm can reduce protons in water (moreprecisely hydronium ions; often briefly termed herein as “H⁺”) to H₂,usually in conjunction with a common catalytic species which whensupplied with electrons can reduce protons to hydrogen, e.g. Pd or Pt.Many other metal-containing complexes too, e.g. noble metal complexes,natural chlorophyll (with Mg as the central atom), Cu-chlorine andCu-2-α-oxymesoisochlorine or other metal-containing phthalocyanines ormetal-containing porphyrines or purely organic compounds having anextended π system, such as among others H₂-chlorine and proflavineexhibit excited states when irradiated with visible light which havesufficient energy to permit reduction of H⁺ to ½H₂ where necessary withfurther conductive transfer of the excited electrons causing chargeseparation to a suitable auxiliary material or catalyst (e.g. Pd, Pt, Ruor a zinc-containing species).

Also a great variety of (where necessary doped) oxide- andoxynitride-containing materials, and, although usually less preferred,phosphide-, arsenide-, antimonide-, sulphide- and selenide-containingmaterials (e.g. SrTiO₃ doped with Cr/Sb or Rh, ZnIn₂S₄, TaON and NiM₂O₆(M=Nb, Ta) can generate H₂ from water (where necessary in the presenceof an auxiliary material or catalyst such as Pt, Pd or Ru) whenirradiated with light ≧420 n nm. In addition, photosemiconductingpolymers can employed exhibiting, where necessary in conjunction with afurther organic non-polymer material, photovoltaic properties.

Exemplary specific photoactive materials are described in examples 3, 4and 5 of the present application.

When employing the photocatalytic system of the invention likewiseeither the second photoactive material or the corresponding auxiliarymaterial or catalyst, if present, or both are in contact with water.

In accordance with the invention the first photoactive material ispreferably not the complete photosystem 2, possibly modified, of plantsor bacteria, (which thereby split water into oxygen and protons). Itpreferably particularly does not comprise polypeptides or proteins. Thereason is that the natural photosystem 2 is very unstable.

In accordance with the invention the second photoactive material it ispresently preferred not to be the complete photosystem 1, possiblymodified, of plants or bacteria (which thereby convert (reduce) NADP⁺into NADPH⁺H or reduce protons to hydrogen by the aid of hydrogenase inspecial cases). It preferably particularly does not comprisepolypeptides or proteins. The reason is that hydrogenase is verysensitive to oxygen and that it might be difficult to couple differentauxiliary catalysts for the reduction of protons to the completephotosystem 1.

Although in principle useful, the first photoactive material ispreferably not a single crystal or derived therefrom by doping. Singlecrystals, doped crystals grown by epitaxy and the like are expensive tomanufacture.

Fro the same reason the second photoactive material is preferably not asingle crystal or derived therefrom by doping, though such material isuseful in principle.

It further is preferred that neither of the photoactive materials isdoped silicon, since this material is also expensive.

Preferably in the present invention, not both photoactive materials i.e.either the first photoactive material or the second photoactivematerial, are conventional semiconductors or photosemiconductors used inphotovoltaics, such as III-V semiconductors, II-VI semiconductors orII-V semiconductors or III-VI semiconductors or similar semiconductorsthat may include mono-, di- and/or trivalent cations of the transitionmetals and group Va and VIa anions of the periodic table of elements. Itis furthermore preferred that none of the two photoactive materials isselected from the above-mentioned semiconductors. Such semiconductorsare in principle useful, however they often are expensive, includemetals that are environmentally harmful and/or are not stable in thepresence of water and in that case may develop poisonous gases.

Preferably the combination of the first and second photoactive materialsis not a combination of a semiconducting oxide absorbing the blue andgreen portion of the solar emission spectrum and of a mesoporousphotovoltaic film using the yellow, red and near infrared portion of thesolar emission spectrum for proton reduction, as long as the twomaterials are arranged in sequence so that yellow, red and near infraredlight not absorbed by the first photoactive material is transmitted tothe second photoactive material. Such an arrangement usually requires aspecific tandem cell built for the cleavage of water which is notpreferred in the present invention.

Preferably the first photoactive material and the second photoactivematerial are different chemical species and furthermore preferably donot consist of the same elements.

The first and second photoactive material can be combined in accordancewith the Z scheme (see above).

When the second photoactive (reduction-promoting) material is irradiatedwith light an electron thereof moves to an excited state from which—whenthe energy is sufficient—it is transferred to protons in the water(often with the aid of an auxiliary material or catalyst, e.g. Pt or Ru)resulting in hydrogen and a photoactive reduction-promoting material orsecond photoactive material with a hole or an oxidation state elevatedby 1, respectively.

The cycle is closed by an excited electron from the first photoactivematerial reduces the oxidized second photoactive material.

In one concrete case by way of example of a suitable Ru(II) complex whenexcited, can transfer its electron (where necessary via an auxiliarymaterial and/or catalyst) to a proton and becomes itself oxidized intothe Ru(III) complex which can abstract, via an electron-conductingmaterial, an electron from a suitable excited oxidation-promotingmaterial, e.g. RuS₂, thereby closing the electrical circuit for cleavingwater. The pH of the water remains constant, since resulting protons arereduced continuously.

To avoid a radiating or radiationless deactivation or electron holerecombination of the excited first and second photoactive material itmay be wanted to provide an adjacent electron acceptor materialeffecting an as much as possible irreversible charge separation which,where necessary, can relay the charge (the electron). Such a materialmay involve without being restricted thereto e.g. nanocrystallinetitanium dioxide or In₂O₃ doped with tin, an organic acceptor compound,such as a quinone or methyl viologen, gold or a further complex compoundwhich can relay electrons e.g. via molecular wires.

In the context of this invention an auxiliary material and/or catalystis defined as a material which either promotes the transfer of electronsfrom oxygen (e.g. RuO₂, Pt, or a compound formed in situ from cobalt ora cobalt containing compound and a phosphate) to the first photoactivematerial, optionally via a conducting or semiconducting material, or thetransfer of electrons from the second photoactive material to protons,optionally via a conducting or semiconducting material (e.g. Au, Pt orRu). However, it is not photoactive itself, i.e. it is unable to effectcleavage of water without an additional photoactive material.

Electron conduction in the catalyst system in accordance with theinvention can be effected with all known electron-conducting materials.Electron-conducting materials are e.g. metals, alloys, semiconductors,conductive oxides, conductive polymers, but also so-called molecularwires (e.g. carbon or hydrocarbon chains or generally covalent boundbranched or unbranched chains in a wealth of differing structures whichmay comprise one or more functional groups and exist in the form ofsubstituents of a chemical compound or independently therefrom and arecapable of conducting electrons) or so-called nanowires, [“wires” havinga diameter of the order of a nanometer (10⁻⁹ meter) including metallic(e.g. Ni, Pt, Au), semiconducting (e.g. Si, InP, GaN etc) and in themacroscopic state isolating materials (e.g. SiO₂, TiO₂), as well asmolecular nanowires composed of repeating units of either an organic(e.g. DNA) or inorganic nature (e.g. Mo₆S_(9-x)I_(x)). The electrons mayalso hop from molecule to molecule in certain material combinations.

In organic compounds or in ligands of complexes one or more of thefunctional groups thereof may be an optionally protected thiol group andthe electron-conducting material to which the optionally protected thiolgroups are bound may comprise gold.

For instance, electron conduction from a first to a second photoactivematerial may take place via the sequence: nanocrystalline titaniumdioxide/indium tin oxide (ITO)/copper/gold/molecular wire. Othersequences are conceivable. The electrons from the second photoactive(reduction-promoting) material can likewise be transferred to the protonor hydroniumion by different ways and means. Electron conduction up tothe proton may involve an auxiliary catalyst, in which case furtherelectron-conducting material(s) may exist between the second photoactivematerial and the auxiliary catalyst.

Preferably the conducting material does not exclusively comprise organicmolecules or complexes of the kind as is to be found in the naturalphotosynthesis system.

“Electrical contact” or “direct electrical contact” in conjunction withthe present invention means material being in electrical contactexclusively via one or more electron-conducting or relaying materials inthe solid state, but not via other materials such as ion conductingmaterials in a fluid or solid medium or liquid redox electrolytes orsolid electrolytes.

When the first (oxidation-promoting) or second (reduction-promoting)photoactive material is an organic molecule or a complex with organicligand(s) the conduction between the two photoactive materials usuallyincludes an electron transition from an organic to an inorganic materialor vice-versa, in the special case of a complex from the central atom ofthe complex via the ligand(s) to the conductive material or from theconductive material via the ligand(s) to the central atom of thecomplex.

This is usually no problem in the transition of an electron from thecentral atom to the ligand, and substituent(s) of the ligand areselected so that they are molecular wires. But the transition of anelectron from the ligand or its substituent(s) for example to aninorganic conductor does not occur directly. Here, good results havebeen attained by introducing functional groups on the ligand or at theend of a ligand substituent capable of interacting with the inorganicmaterial so strongly that electron conduction is possible. A primeexample thereof is binding thiols to gold surfaces, although there is awealth of other such interactions, e.g. those of phosphonic acids,carbon acid anhydrides or silanes to inorganic oxides (see e.g. anreview thereof in the article by Elena Galoppini “Linkers for anchoringsensitizers to semiconductor nanoparticles” Coordination ChemistryReviews 2004 248, 1283-1297).

A complex being the second photoactive material may comprise at leasttwo functional groups, of which at least one is bound to theelectron-conducting materials and the other is bound to a furtherelectron-conducting material comprising a chemical species which canreduce protons into water when electrons are supplied.

Some photoactive materials, especially reduction-promoting complexes,are unstable in the simultaneous presence of light, water and oxygen.The present invention proposes a way of avoiding such instability by“casketizing” such, material in a transparent inorganic “casket” withthe exclusion of water and oxygen, e.g. in a thin transparent “goldcasket” with insulating sidewalls. In the latter case e.g. areduction-promoting complex may comprise at least two functional groups,e.g. thiol groups, one of which, as explained above, serves to ensureelectron conduction via the “floor” of the “casket” to theoxidation-promoting photoactive material, and the other serves toprovide the conduction e.g. to an alloy type of material, e.g.gold/platinum (the “casket lid”) that catalyzes the production ofhydrogen with the aid of the excited electrons released from the centralatom of the complex which are relayed via molecular wires to thisalloy-type material. It will, of course, be appreciated that other such“casket” or “sandwich” structures are equally suitable.

The first (oxidation-promoting) photoactive material and the second(reduction-promoting) photoactive material may be mounted on orotherwise connected with one or more substrates, e.g. by physicaldeposition or some kind of by chemical bonding. The substrate may alsobe coated with an electrically conductive material, on which or withwhich the first (oxidation-promoting) photoactive material and thesecond (reduction-promoting) photoactive material may be mounted orotherwise connected, e.g. by physical or chemical deposition or somekind of by chemical bonding. The substrates may be electrically andphoto-chemically inert, or not, and may be transparent or translucent(for instance glass) to permit the passage of light not absorbed by thephotoactive material directly irradiated, or not. Non-limiting examplesfor the material of the substrate are optionally coated glass, ceramics,metal or metal alloys, semimetals, carbon or materials derived fromcarbon and all kinds of inorganic and organic polymeric materials.

With the aid of such a substrate a plane, e.g. plate-shaped, or also atubular or otherwise appropriately shaped catalyst system can beconstructed, e.g. with the photoactive oxidation-promoting material onone side and the photoactive reduction-promoting material on the otherside, but also, when suitably structured, also with both materials onthe same side. When the substrate is transparent or translucent it maybe sufficient to irradiate one side of a plate-type catalyst system toalso supply light to the photoactive material at the other side.

When a plane catalyst systems having the photoactive materials onopposite sides, for instance when plate-shaped, are immersed in anaqueous fluid, hydrogen is generated on one side and oxygen on theother. The way in which this is achieved already makes for hydrogen andoxygen being separated spatially, greatly diminishing the risk of anoxygen-hydrogen reaction. Totally separating the hydrogen from theoxygen is achievable by engineering the two photoactive materialstotally separated from each other spatially, as is possible bycompartmenting a reactor or reactor system into two chambers or into2-chamber systems by means of a material exclusively permeable forprotons and water (e.g. a Nafion® membrane). Protons must be able todrift to and fro between both chambers to compensate the charge.

The aqueous fluid into which the plane e.g. plate-type catalyst systemof the present invention is immersed is normally water which maycontain, depending on the case concerned, all kinds of soluble salts,acids or bases, but not by necessity. And, of course, e.g. mixtures ofsolvents and surfactants and the like soluble in water and, wherenecessary, watery emulsions and the like not involved in the photolysisreaction are a possible medium should it prove necessary, as long as thephotolysis of the water is not disturbed or prevented thereby.

In the method of the invention the light used for irradiating thecatalyst systems is preferably sunlight.

Furthermore, the first location and the second location irradiated arepreferably separated from each other by a membrane permeable only forprotons and water, e.g. a Nafion® membrane.

Only the first location of the catalyst system may be directlyirradiated with light e.g. If the system is sufficiently transparent orpartly transparent. Alternatively, only the second location may directlyirradiated with light. In many cases, both locations are directlyirradiated with light.

Oxygen and/or hydrogen evolving from water with the aid of the catalystsystem and light may be intermittently or continuously collected.

The photocatalyst system in accordance with the invention has manyadvantages. Hydrogen and oxygen can be generated separately withoutproduction of oxygen-hydrogen gas. The system does not take the form ofa powder but is monolithic, e.g. in the form of a plate which is simplyimmersed in an aqueous medium, requiring often no addition of any salts,acids or bases (although this is not excluded) which possibly add to thecost or environmental load of the method, all without the need of anyspecial cells needing to be pressurized or involving a redox electrolytewhich has to be encapsulated solvent-proof. The system is extremelyflexible, featuring a large choice of water oxidizing catalysts (firstphotoactive materials) and water (or protons) reducing catalysts (secondphotoactive materials) enabling suitable combinations to be tailor-made.

Structure of an Exemplary Catalyst System

FIG. 2 depicts a diagrammatic cross-section through the configuration ofa photocatalyst system 1 working analogously to the Z scheme, whichfeatures on one side of an inert plate-type substrate 10 a transparentconductive layer coated with indium doped tin oxide (ITO) 30, on theother side a transparent layer of gold 40. The ITO layer 30 and goldlayer 40 are electrically connected by copper bands 20.

Sintered on the ITO layer 30 is nanocrystalline TiO₂ 50 coated withRuS₂. Provided on the gold layer 40 is a monolayer of a rutheniumcomplex 60 (depicted far too thick) with three mercaptoalkylsubstituents 60 a (see example III.2.b) and an alkylthiol 70. The edgesof the monolayer are framed on all sides by a resist 80 extending overthe edge of the monolayer and covering the monolayer with a narrow bandof resist. Vacuum deposited on the monolayer of the ruthenium complex 60and alkylthiol 70 is a transparent thin gold layer 90 comprising just afew layers of gold and extending beyond the resist 80. Over the goldlayer 90 a platinum layer 92 with fewer atoms of platinum than of amonolayer is deposited.

When the catalyst system as shown in FIG. 2 is immersed in water andirradiated with light having a wavelength≧420 nm electrons originatingfrom the oxygen atoms of the H₂O which has been oxidized to ½O₂+2H⁺migrate from the TiO₂ 50 coated with ruthenium disulfide via the ITOlayer 30 and copper bands 20 to the gold layer 40. Since the rutheniumcomplex 60 on being irradiated has given off an electron to an H⁺ anelectron migrates from the ruthenium complex 60 via the thiol group andthe alkyl chain of the mercaptoalkyl substituents 60 a of the rutheniumcomplex 60 to its Ru central atom. Irradiation of the ruthenium complex60 causes it excitation to give off an electron to the gold layer 90coated with platinum 92 via the alkyl chain and the thiol group of afurther mercaptoalkyl substitute 60 a and from there via the platinum 92to a proton (H⁺) in water which is reduced thereby to ½H₂.

EXAMPLES

The invention is further illustrated by the following non-limitingexamples.

Example 1 A. Preparation of a Oxidation-Promoting First PhotoactiveMaterial on TiO₂ in the Form of a 5% Suspension of TiO₂/RuS₂ (2% byWeight RuS₂ Relative to TiO₂)

Five grams of an aqueous TiO₂ suspension (10%, Aldrich) are diluted with15 ml water and added with 23 mg (0.11 mmol) ruthenium(III)-chloride(RuCl₃), treated in an ultrasonic bath and concentrated under reducedpressure until dry.

The RuCl₃ deposited on the TiO₂ powder is firstly reduced to the metal(Ru) under an inert gas atmosphere in a stream of hydrogen gas (H₂). Forthis purpose the samples are heated to 300° C. and treated for 3 h in aflow of hydrogen at a rate of 50 ml/min. Then the temperature iselevated to 400° C. and 10 ml/min hydrogen sulfide are admixed; thisinitiates the sulfidation of Ru into black ruthenium sulfide (RuS₂)which is continued for a further 4 h [A. Ishiguro, T. Nakajima, T.Iwata, M. Fujita, T. Minato, F. Kiyotaki, Y. Izumi, K.-i. Aika, M.Uchida, K. Kimoto, Y. Matsui, Y. Wakatsuki, Chem. Eur. J. 2002, 8 (14),3260-3268./K. Hara, K. Sayama, H. Arakawa, Appl. Catal. A.: Gen. 1999,189 (1), 127-137]. This results in a gray powder which is then admixedin a quantity of 50 mg with 1 ml water and sufficiently suspended in theultrasonic bath to give a light gray suspension of RuS₂ (2% by weight)on TiO₂.

B. Applying the Above Oxidation-Promoting First Photoactive Material toan ITO Substrate

Commercially available glass slides coated on one side with indium tinoxide (ITO) (available from PGO Präzisions Glas & Optik GmbH, Im LangenBusch 14, D-58640 Iserlohn, Germany) are thinly coated on the ITO sidewith an aqueous 10% TiO₂ suspension (from Aldrich, particle size <40 nm)and sintered for 60 min at 450° C., after which the aqueous 5%suspension of TiO₂/2% by weight RuS₂ as prepared above is coated and theslides resintered for 60 min at 450° C. under an inert gas atmosphere.

The resulting slide is designated Ox-I.

Example 2 A. Preparation of Photoactive WO₃ Nanoparticles and theirPlatinized Form as Oxidation-Promoting First Photoactive Materials

The preparation of photoactive WO₃ nanoparticles was performed accordingto literature procedures (J. Polleux, M. Antonietti, M. Niederberger, J.Mater. Chem. 2006, 16 (40), 3969-3975./M. Niederberger M. H. Bartl, G.D. Stucky, J. Am. Chem. Soc. 2002, 124 (46), 13642-13643./J. Polleux, N.Pinna, M. Antonietti, M. Niederberger, J. Am. Chem. Soc. 2005, 127 (44),15595-15601.)

In a typical experiment tungsten hexachloride (WCl₆, 430 mg) wasdissolved in 20 ml of anhydrous benzyl alcohol (or a mixture thereofwith 4-tert.-butyl-benzylalcohol). The closed reaction vessel was heatedat 100° C. with stirring for 48 hr. The product was collected byalternating sedimentation and decantation and washed three times with 15ml EtOH. The material obtained was dried in air at 60 C for severalhours to yield a yellow powder of WO₃.

For an optional platinization, 50 mg of powder was suspended in amixture of ethanol (50%) and water (50%). Pt cocatalyst (2% weight perWO₃) was deposited from a neutralized aqueous solution of H₂PtCl₆.6H₂Oby a photodeposition method (K. Yamaguti, S. Sato, J. Chem. Soc. FaradayTrans 1 1985, 81 (5), 1237-1246./T. Sakata, T. Kawai, K. Hashimoto,Chem. Phys. Lett. 1982, 88 (1), 50-54.)

B. Applying the Above Oxidation-Promoting First Photoactive Materials toan ITO Substrate

Quantities of 20 mg dry powder of platinized (grey) or non-platinized(yellow) catalyst were resuspended by ultrasonication in a mixture of0.4 ml abs. isopropanol and 0.2 ml water (Suprapur). Small aliquots ofeach suspension were deposited on appropriate ITO-coated glass slides,respectively. The catalyst coated slides were air dried for 15 min andsubsequently sintered at 450 h for 2 hr.

The resulting slide coated with plain WO₃ is designated Ox-IIa.

The resulting slide coated with platinized WO₃ is designated Ox-IIb.

Example 3 A. Preparation of the Mn₄O₄ Oxo-Cubane ComplexMn₄O₄(Phenyl₂PO₂) as a Oxidation-Promoting First Photoactive Material

[On the basis of literature procedures: R. Brimblecombe, G. F. Swiegers,G. C. Dismukes, L. Spiccia, Angew. Chem. Int. Ed. 2008, 47 (38),7335-7338./T. G. Carrell, S. Cohen, G. C. Dismukes, J. Mol. Cat. A 2002,187 (1), 3-15.]

A solution of 60 mg NaOH (1.5 mmol) in 20 ml DMF is provided under inertgas atmosphere (N₂). 330 mg diphenyl phosphinic acid (1.5 mmol) and 255mg manganese(II) perchlorate (0.7 mmol) dissolved in 8 ml DMF are addedto the solution with vigorous stirring. After a reaction period of 15min 50 mg KMnO₄ (0.3 mmol) dissolved in 18 ml DMF are slowly addeddropwise through an addition funnel. A brownish red suspension isformed, which is stirred for 16 hr at RT. Die suspension is filtert, theresidue washed with each of 40 ml of methanol and ether and dried. Thetitle complex is obtained in the form of a brownish red powder.

UV/Vis (CH₂Cl₂): λ_(max) (Ig ε)=229.0 (0.80), 263.0 (0.36), 257.0(0.36), 269.5 (0.34).

UV/Vis spektrum: see FIG. 5

B. Applying the Above Oxidation-Promoting First Photoactive Material toan ITO Substrate in a Nafion® Matrix

The application of the above Mn₄O₄(phenyl₂PO₂) complex to an ITO-coatedglass slide was effected on the basis of the following literatureprocedures: M. Yagi, K. Nagai, A. Kira, M. Kaneko, J. Electroanal. Chem.1995, 394 (1-2), 169-175.

Commercially available glass slides coated on one side with indium tinoxide (ITO) (available from PGO Präzisions Glas & Optik GmbH, Im LangenBusch 14, D-58640 Iserlohn, Germany) are thinly coated on the ITO sidewith a 1 mM solution of the Mn₄O₄(phenyl₂PO₂) complex which wasdissolved in a 1:1 mixture of Nafion® 117 solution and abs. Ethanol anddried for 12 hr in air.

The resulting slide is designated Ox-III.

Example 4 A.Tris[4-(11-mercaptoundecyl)-4′-methyl-2,2′-bipyridine]ruthenium(II)-bis-(hexafluorophosphate),a reduction-promoting second photoactive material A.1 Preparation of theprotected bipyridine ligand4-(4′-methyl-2,2′-bipyridine)-undecylthio-S-acetate (B)

[Analogous to D. K. Ellison, R. T. Iwamato, Tet. Lett. 1983, 24 (1),31-32./P. K. Gosh, T. G. Spiro, J. Am. Chem. Soc. 1980, 102 (17),5543-5549.]

a) 4-(11-bromoundecyl)-4′-methyl-2,2′-bipyridine (A)

100 ml abs. tetrahydrofurane (THF) are cooled to 0° C. in an inert gasatmosphere (N₂) and mixed with 2.00 g (10.9 mmol) of4,4′-dimethyl-2,2′-bipyridine. After stirring for 15 min at 0° C. 5.45ml (10.9 mmol) of a 2 M solution of lithium diisopropylamide (LDA) areadded to the THF which is then reacted for 2 hr with cooling. Thereaction solution is added dropwise over a period of 30 min to asolution of 3.30 g (11.0 mmol) of 1.10 dibromodecane in 30 ml abs. THFat 0° C. The brownish clear solution is stirred for a further 2 hr at 0°C. and for 16 hr at RT before being quenched with 10 ml H₂O. Thesuspension is then concentrated under a reduced pressure almost untildry. The aqueous residue is taken up in 25 ml of water, and 25 ml brineis added and the mixture extracted three times with 75 ml CHCl₃ each.The organic phases are dried over Na₂SO₄, filtered and concentrateduntil dry under a reduced pressure. Purifying of the product mixture isachieved on silica gel with ethyl acetate (EtOAc). The product A isobtained in the form of a beige crystalline powder.

b) 4-(4′-Methyl-2,2′-bipyridine)-undecylthio-S-acetate (B)

[Analogous to H. Imahori, A. Fujimoto, S. Kang, H. Hotta, K. Yoshida, T.Umeyama, Y. Matano, S. Isoda, Tetrahedron 2006, 62 (9), 1955-1966.]

Under an inert gas atmosphere (N₂) a solution of 1.20 g of A (2.97 mmol)in abs. ethanol (EtOH) and abs. THF (50 ml, 1/1, V/V) is combined with2.04 g thioacetate (6 equ., 17.9 mmol) and refluxed for 2 hr. Aftercompletion of the reaction the reddish-brown clear solution isconcentrated at a reduced pressure. The residue is then taken up in 50ml of chloroform, washed twice with 40 ml of water each and once with 40ml brine, dried over Na₂SO₄, filtered and the solvent removed at areduced pressure. The residue is purified on silica gel withEtOAc/n-hexane (1:1). Product B is obtained as a yellow solid.

4-(4′-Methyl-2,2′-bipyridine)-undecylthio-S-acetate (B)

¹H-NMR (300 MHz, CDCl₃): δ=8.53 (dd, 2H, J_(5/6)=J_(5′/6′)=4.2 Hz,J_(3/6)=J_(3′/6′)=0.6 Hz, 6-H, 6′-H), 8.21 (br s, 2H, 3-H, 3′-H), 7.14(dd, 2H, J_(5/6)=J_(5′/6′)=4.8 Hz, J_(3/5)=J_(3′/5′)=1.5 Hz, 5-H, 5′-H),2.84 (t, J=7.5 Hz, 2H, 17-CH₂, S—CH₂), 2.67 (t, J=7.8 Hz, 2H, 7-CH₂,Aryl-CH₂), 2.42 (s, 3H, 7′-CH₃, Aryl-CH₃), 2.30 (s, 3H, 20-CH₃, CO—CH₃),1.72-1.62 (m, 2H, 8-CH₂), 1.59-1.49 (m, 2H, 16-CH₂), 1.38-1.24 (m, 14H,9/10/11/12/13/14/15-CH₂).

¹³C-NMR (75.5 MHz, CDCl₃): δ=196.0 (C-19), 156.1 (C-2), 156.0 (C-2′),152.9 (C-4), 148.9 (C-6), 148.9 (C-6′), 148.1 (C-4′), 124.6 (C-3′),123.9 (C-3), 122.0 (C-5′), 121.2 (C-5), 35.5 (C-7), 30.6 (C-17), 30.4(C-8), 29.4 (C-11/12), 29.4 (C-10), 29.4 (C-13), 29.3 (C-15), 29.3(C-9), 29.1 (C-16), 29.0 (C-20), 28.8 (C-14), 21.2 (C-7′).

EI-MS (70 eV): m/z (%)=398 (4) [M⁺], 357 (18), 356 (68), 355 (90)[M⁺-Me, —CO], 324 (9), 323 (39), 309 (18), 295 (12), 281 (10), 267 (6),253 (6), 239 (6), 211 (13), 209 (5), 198 (21), 197 (100), 185 (12), 184(95), 183 (10), 170 (5), 43 (30), 41 (5).

UV/Vis (MeOH): λ_(max) (Ig ε)=236.5 (0.74), 280 (0.71).

A.2 Preparation ofTris[4-(11-mercaptoundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-bis(hexafluorophosphate)(C) a)Tris[4-(4′-methyl-2,2′-bipyridine)-undecylthio-S-acetate]-ruthenium(II)-bis(hexafluorophosphate)(protected C)

[Analogous to R. A. Palmer, T. S. Piper, Inorg. Chem. 1966, 5 (5),864-878. und Y-R. Hong, C. B. Gorman, J. Org. Chem. 2003, 68 (23),9019-9025.]

Under an inert gas atmosphere (N₂) 30 ml of abs. ethanol flushed with N₂are combined with 180 mg (452 μmol/25% excess relative to RuCl₃) of B asprepared in Example 4.A.b. To reaction solution 25 mg of RuCl₃.H₂O(120.5 μmol) in 5 ml abs. ethanol are added and refluxed for 65 hr inthe dark. After completion of the reaction the reddish-brown suspensionis filtered and the clear reddish-orange filtrate is concentrated on arotational evaporator at a reduced pressure. The red residue taken up in20 ml of CH₂Cl₂ and washed twice with 20 ml of water each. The organicphase is dried over Na₂SO₄, filtered and concentrated on a rotationalevaporator at a reduced pressure. The resulting reddish-brown productmixture is purified on silica gel with CHCl₃/methanol (15/1) to obtainthe title compound.

b)Tris[4-(11-mercaptoundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-bis-(hexafluorophosphate)(C)

[Analogous to H. Imahori, A. Fujimoto, S. Kang, H. Hotta, K. Yoshida, T.Umeyama, Y. Matano, S. (soda, Tetrahedron 2006, 62 (9), 1955-1966./F.Ono, S. Kanemasa, J. Tanaka, Tetrahedron Lett. 2005, 46 (44),7623-7626./T. Suzuki, A. Matsuura, A. Kouketsu, S. Hisakawa, H.Nakagawa, N. Miyata, Bioorg. Med. Chem. 2005, 13 (13), 4332-4342.]

In a 100 ml three-neck round bottom flask 10 ml abs. ethanol are flushedfor 30 min with N₂. Together with a further 10 ml of abs THF 44 mg (32.2μmol) of the protected Ru complex as prepared above in section A.2.a atRT are transferred into the flask and combined with 100 mg KOH (55 eq.).The reaction solution is stirred for 60 min at RT, then poured onto 15ml of brine and extracted with 25 ml of CH₂Cl₂. The organic phase isbriefly dried over Na₂SO₄, filtered and concentrated at a reducedpressure. The red complex is combined with a solution of 105 mg NH₄ PF₆(0.6 mmol, ˜20 eq.) in 5 ml methanol and stirred for 120 min at RT inthe dark. The red solution is washed twice with water, dried over alittle Na₂SO₄, filtered and stripped in a rotational evaporator at areduced pressure. The residue is dissolved in a little CH₂Cl₂ andtriturated with petrol ether. The title Ru complex C precipitates as anorange-red powder.

Tris[4-(11-mercaptoundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-bis(hexafluorophosphate)(C)

¹H-NMR (300 MHz, CDCl₃): δ=8.17 (d, 2H*3, J=7.8 Hz, 6-H*3, 6′-H*3),7.58-7.45 (m, 2H*3, 3-H*3, 3′-H*3), 7.23 (dd, 2H*3 J=5 Hz, J=2 Hz,5-H*3, 5′-H*3), 2.79 (t, 2H*3, J=8 Hz, 7-CH₂, Aryl-CH₂), 2.53 (s, 3H*3,7′-CH₃, Aryl-Me), 2.50 (t, 2H*3, J=7.2 Hz, 17-CH₂, S—CH₂), 1.72-1.62 (m,2H*3, 8-CH₂), 1.61-1.54 (m, 2H*3, 16-CH₂), 1.32 (t, 3*1H, J=7.5 Hz, —SH)1.38-1.23 (m, 14H*3, 9/10/11/12/13/14/15-CH₂).

¹³C-NMR (75.5 MHz, CDCl₃): δ=156.4 (C-2), 156.3 (C-2′), 154.5 (C-4),150.8 (C-6), 150.6 (C-6′), 150.0 (C-4′), 129.0 (C-3′), 128.0 (C-3),124.6 (C-5′), 123.8 (C-5), 35.4 (C-7), 34.0 (C-16), 30.0 (C-8), 29.5(C-10), 29.4 (C-11/12), 29.4 (C-13), 29.3 (C-15), 29.0 (C-9), 28.3(C-14), 24.6 (C-17), 21.3 (C-7′).

ESI-MS: m/z=1315.6 ([M-(PF₆)]⁺), 585.3 ([M-(2*PF₆)]²⁺).

UV/Vis (MeOH): λ_(max)(Ig ε)=286.5 (1.27), 248.0 (0.36), 258.0 (0.35),457.5 (0.26), 436 (sh, 0.22), 324.5 (0.20).

UV/Vis spectrum: see FIG. 3

B. Applying the Reduction-Promoting Second Photoactive Material C to anInert Substrate

Glass slides of the same size as used for applying the first photoactivematerial to glass slides and having a 50 nm gold coating on one of thesides precoated with 5-10 nm chromium are treated with a solution oftris[4-(11-mercaptoundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-bis(hexafluorophosphate)(C) in ethanol. In one embodiment the coating the complex in solution isperformed in the presence of an alkylthiol having a chain length C8,C10, C12, C14, C16 or C18 as a coadsorbate analogous to S. J. Summer, S.E. Creager, J. Am. Chem. Soc. 2000, 122 (48), 11914-11920. To increasethe solubility of the complex the solution where necessary is combinedwith a little dichloromethane. In another embodiment, the coating isperformed without coadsorbate analogous to Y. S. Obeng, A. J. Bard,Langmuir 1991, 7 (1), 195-201. After coating, the slides are dried.

The resulting slide is designated Red-I.

Example 5 A. Preparation of[(2,2′-bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-[(tetrapyridophenazine)-palladium(II)-dichloro]bis-(hexafluorophosphate)(H) as a reduction-promoting second photoactive material

(Abbreviations used for the ligands: bpy=2,2′-bipyridine,Me-bpy=4′-methyl-2,2′-bipyridine, tppz=tetrapyridophenazine)

The following preparations are performed in analogy to P. A. Anderson,G. B. Deacon, K. H. Haarmann, F. R. Keene, T. J. Meyer, D. A. Reitsma,B. W. Skelton, G. F. Strouse, N. C. Thomas, J. A. Treadway, A. H. White,Inorg. Chem. 1995, 34 (24), 6145-6157.

A.1 Preparation of cis-dicarbonyl(2,2′-bipyridine)[4-(11-acetylsulfanyl-undecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-bis(hexafluorophosphate)(E)

195 mg 4-(4′-Methyl-2,2′-bipyridine)-undecylthio-5-acetate (B) (0.49mmol; prepared in Example 4, section 4.1.b) are dissolved in 30 ml abs.ethanol under an inert atmosphere (N₂). 150 mgcis,cis-[Ru(bpy)(CO)₂(CF₃SO₃)₂] (D) (0.25 mmol; prepared according to P.A. Anderson, G. B. Deacon, K. H. Haarmann, F. R. Keene, T. J. Meyer, D.A. Reitsma, B. W. Skelton, G. F. Strouse, N. C. Thomas, J. A. Treadway,A. H. White, Inorg. Chem. 1995, 34 (24), 6145-6157) are added to thesolution and heated for 2 hr at reflux. The clear reddish solution isconcentrated under a reduced pressure until dry. The residue isdissolved in 25 ml of water and filtered. After cooling to RT 5 ml of anaqueous solution of NH₄ PF₆ saturated when cold is added to the filtrateimmediately yielding a colorless precipitate. The precipitate isfiltered off and washed with 30 ml of cold water. The title product (E)is obtained as a pink solid which is recrystallized in littleethanol/acetone.

cis-Dicarbonyl(2,2′-bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-bis(hexafluorophosphate)(E)

Compound E exists in two geometric isomers A/B [T. J. Rutherford, F. R.Keene, Inorg. Chem. 1997, 36 (13), 2872-2878.]

¹H-NMR (300 MHz, (CD₃)₂CO): δ=9.50 (m, H, Aryl-H), 9.31 (dd, H, J=6.0Hz, J=8.0 Hz, Aryl-H), 8.94 (d, H, J=8.4 Hz, Aryl-H), 8.85 (m, H,Aryl-H), 8.82 (d, H, J=8.4 Hz, Aryl-H), 8.73 (m, H, Aryl-H), 8.63 (m, H,Aryl-H), 8.39 (m, H, Aryl-H), 8.11 (m, H, Aryl-H), 7.96 (m, H, Aryl-H),7.82 (m, H, Aryl-H), 7.69 (m, H, Aryl-H), 7.65 (t, H, J=6.0 Hz, Aryl-H),7.52 (m, H, Aryl-H), 3.04/2.82 (t, 2H, J=8 Hz, 7-CH₂ A/B), 2.85/2.83 (t,2H, J=7.2 Hz, 17-CH₂ A/B), 2.75/2.53 (s, 3H, Aryl-CH₃ A/B), 2.29/2.28(s, 3H, CO—CH₃ A/B), 1.86/1.65 (m, 2H, 8-CH₂ A/B), 1.57-1.45 (m, (m, 2H,16-CH₂ A/B), 1.31-1.25 (m, 14H, 9/10/11/12/13/14/15-CH₂ A/B).

EI-MS (70 eV): m/z (%)=713 (8) [M-H⁺-2*PF₆], 616 (12), 506 (9), 449(14), 353 (22), 322 (35), 197 (100), 184 (88), 156 (28), 65 (17).

A.2 Preparation of (2,2%bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-(tetrapyridophenazine)-bis(hexafluorophosphate)(G)

145 mg cis-dicarbonyl(2,2′-bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-bis(hexafluorophosphate)(E) (145 μmol; prepared above in section A.1) are dissolved in 40 ml ofabs. 2-methoxyethanol. 165 mg tetrapyridophenazine (F) (tppz; 0.43 mmol,3 eq.; prepared according to W. Paw, R. Eisenberg, Inorg. Chem. 1997, 36(11), 2287-2293./J. Bolger, A. Gourdon, E. Ishow, J.-P. Launay, J. Chem.Soc., Chem. Commun. 1995, 1799-1800). in a further 10 ml of abs.2-methoxyethanol are added thereto. The yellow suspension iscontinuously gently flushed with N₂. After heating to 100° C. andpurging with N₂ for about 10 min 56 mg trimethylamine-N-oxide (ZMNO, 0.5mmol, ˜3.5 eq.) dissolved in 5 ml of abs. 2-methoxyethanol are added.While purging continuously with N₂ the suspension is refluxed for 24 hrin the dark. After completion of the reaction the reddishbrown-suspension is concentrated to half of its volume and left for 30min in the dark. The settled non-reacted tppz is filter off. The residuein the filter is washed with 15 ml of ethanol and the deep red filtrateis concentrated under a reduced pressure. A deep red, almost black oilis obtained. After chromatographic separation and purification on silicagel with CHCl₃/methanol (5:1) the title product (G) is isolated as aorange-red enamel-like solid.

(2,2′-Bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-(tetrapyridophenazine)-bis(hexafluorophosphate)(G)

ESI-MS (CH₃CN): m/z=566.2 ([M-(2*PF₆)+HCOOH]²⁺).

UV/Vis (CH₃CN): λ_(max) (Ig ε)=282.0 (1.02), 244.0 (0.60), 379.5 (0.19),359.0 (0.16), 453.5 (0.16), 434.0 (sh, 0.15).

A.3 Preparation of[(2,2′-bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-[(tetrapyridophenazine)-palladium(II)-dichloro]bis(hexafluorophosphate)(H)

The reaction is performed in analogy to the procedure of S. Rau, B.Schäfer, D. Gleich, E. Anders, M. Rudolph, M. Friedrich, H. Görls, W.Henry, J. G. Vos, Angew. Chem. Int. Ed. 2006, 45, 6215-6218.

6 mg of bis(acetonitrile)-palladium(II)-dichloride (23 μmol, 15% excess;Aldrich) is combined with 20 ml of abs. dichloromethane (DCM) in aninert gas atmosphere (N₂) and 26 mg of(2,2′-bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-(tetrapyridophenazine)-bis(hexafluorophosphate)suspended in 2 ml of abs. DCM is added. The reaction mixture is heatedto reflux for 6 hr under N₂. After completion of the reaction thesuspension is cooled to RT, filtered, and the reddish-brown residue iswashed with little DCM. After drying the title product is obtained asreddish-brown solid.

[(2,2′-Bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-[(tetrapyridophenazine)-palladium(II)-dichloro]-bis(hexafluorophosphate) (H)

ESI-MS (CH₃CN): m/z=673.2 ([M-(2*PF₆)]²⁺+HCOOH+H₂O]).

FAB-MS (−): m/z=1526.2 (MH⁻+H₂O).

UV/Vis (CH₃CN): λ_(max) (Ig ε)=241.0 (1.34), 283.5 (0.56), 354.5 (sh,0.10), 432.0 (0.08), 453.5 (0.08).

UV/Vis spectrum: see FIG. 4

B. Applying the Reduction-Promoting Second Photoactive Material H to anInert Substrate

The coating of the complex H is performed in analogy to S. J. Summer, S.E. Creager, J. Am. Chem. Soc. 2000, 122 (48), 11914-11920/S. E. Craeger,G. K. Rowe, J. Electroanal. Chem. 1994, 370 (1-2), 203-211 (coating inCH₃CN)/J. M. Tour, L. Jones II, D. L. Pearson, J. J. S. Lamba, T. P.Burgin, G. M. Whitesides, D. L. Allara, A. N. Parikh, S. V. Atre, J. Am.Chem. Soc. 1995, 117 (37), 9529-9534./L. Cai, Y. Yao, J. Yang, D. W.Price jr., J. M. Tour, Chem. Mater. 2002, 14 (7), 2905-2909 (coatingwith acetylsulfanyl group).

Glass slides of the same size as used for applying the first photoactivematerial to glass slides and having a 50 nm gold coating on one of thesides precoated with 5-10 nm chromium are treated with a solution of thecomplex H in CH₃CN. The coating is performed in the presence of acoadsorbate (a C12 or C16 alkylthiol) as well as without coadsorbate.After coating the slides are washed and dried.

The resulting slide is designated Red-II.

Example 6 A. Preparation of Platinized SrTiO₃ Codoped with 2.5% Antimonyand 2.0% Chromium as a Reduction-Promoting Second Photoactive Material

The preparation of the photoactive codoped SrTiO₃ was performedaccording to literature procedures (H. Kato, A. Kudo, J. Phys. Chem. B2002, 106 (19), 5029-5034.)

Appropriate quantities of the starting materials SrCO₃, TiO₂, Sb₂O₃ andCr₂O₃ according to the formula SrTi_(1-X-Y)Sb_(X)Cr_(Y)O₃ wereextensively mixed in a mortar. The mixture was transferred in an aluminacrucible and calcined at 1050° C. for at least 30 h in air. The materialobtained was ball milled (GERMATECH GmbH, Osterfeldstr. 3, D-56235Ransbach-Baumbach, Germany) until a finely ground powder was obtained.

For platinization the powder was suspended in a mixture of ethanol(20%), water (80%). Pt cocatalyst (2% weight per WO₃) was deposited froma neutralized aqueous solution of H₂PtCl₆.6H₂O by a photodepositionmethod (K. Yamaguti, S. Sato, J. Chem. Soc. Faraday Trans 1 1985, 81(5), 1237-1246./T. Sakata, T. Kawai, K. Hashimoto, Chem. Phys. Lett.1982, 88 (1), 50-54.)

B. Applying the Above Reduction-Promoting Second Photoactive Material toan ITO Substrate

100 mg of dry powder of the platinized SrTiO₃ codoped with 2.5% antimonyand 2.0% chromium were resuspended by ultrasonication in a mixture of 2ml abs. ispropanol and 1 ml water (Suprapur). Small aliquots of thesuspension were deposited on commercially available glass slides coatedon one side with indium tin oxide (ITO) (available from PGO PräzisionsGlas & Optik GmbH, Im Langen Busch 14, D-58640 Iserlohn, Germany). Thecatalyst-coated plates were air dried for 15 min and subsequentlysintered for 2 hr at 450 h.

The resulting slide is designated Red-III.

Example 7 Combination of the Catalyst Units Comprising the First andSecond Photoactive Materials into a Catalyst System and Irradiation7.1.a Combining Ox-I and Red-I

Both of the catalyst units (slide Ox-I and slide Red-I) produced inExample 1 and Example 4, respectively, comprising theoxidation-promoting first photoactive material RuS₄ and thereduction-promoting second photoactive materialtris[4-(11-mercaptoundecyl)-4′-methyl-2,2′-bipyridine]ruthenium(II)-bis-(hexafluorophosphate),respectively, are bonded together by their non-coated faces and thecoated surfaces of the two units are conductively interconnected by acopper conductive adhesive tape (made by PGO Präzisions Glas & OptikGmbH, Im Langen Busch 14, D-58640 Iserlohn, Germany). The gold side ofRed-I is then coated with a resist so that the Cu bands and edges of thegold layer and a small band along the edges of the slide containing thecomplex on the gold layer are covered. After having dried the assembledand conductively connected catalyst units the gold side coated with thecomplex is again vapor deposited with a very thin gold layer (5 nm) sothat also the adjoining resist layer is covered finally, the catalystsystem is completed by coating this gold layer with 0.5-0.7 monolayers(ML) of platinum.

7.1.b Irradiating the catalyst system (Ox-I)-(Red-I)

A catalyst system as made in Section 7.1.a was adhered to a Nafion®membrane and a window was cut into the membrane so that both sides ofthe system were exposed. The membrane comprising the catalyst system wasinserted into a photoreactor so that two compartments were formed. Bothcompartments were filled in deoxygenized water saturated with N₂ andcovered with an oil layer. Into the compartment adjacent to Ox-I yellowleucoindigo carmine was introduced. Then the catalyst system wasirradiated from both sides with a 500 Watt halogen lamp through 420 nmcut-off filters. After heating to about 50° C. hydrogen and oxygendeveloped as evident by the change in color of the yellow leucoindigocarmine to the blue indigo carmine (a sensitive indicator for oxygen).

Experiments employing D₂O and H₂ ¹⁸O and using a nitrite solution asfilter confirmed the cleavage of water since D₂ and higher thannaturally occurring quantities of ¹⁶O¹⁸O were demonstrated by massspectrometry.

7.2.a Combining Ox-I and Red-II

Both of the catalyst units (slide Ox-I and slide Red-II) produced inExample 1 and Example 5, respectively, comprising theoxidation-promoting first photoactive material RuS₄ and thereduction-promoting second photoactive material[(2,2′-bipyridine)[4-(11-acetylsulfanylundecyl)-4′-methyl-2,2′-bipyridine]-ruthenium(II)-[(tetrapyridophenazine)-palladium(II)-dichloro]bis-(hexafluorophosphate),respectively, are bonded together by their non-coated faces and thecoated surfaces of the two units are conductively interconnected by acopper conductive adhesive tape (made by PGO Präzisions Glas & OptikGmbH, Im Langen Busch 14, D-58640 Iserlohn, Germany).

7.2.b Irradiating the Catalyst System (Ox-I)-(Red-II)

A catalyst system as made in Section 7.2.a was adhered to a Nafion®membrane and a window was cut into the membrane so that both sides ofthe system were exposed. The membrane comprising the catalyst system wasinserted into a photoreactor so that two compartments were formed. Bothcompartments were filled in deoxygenized water saturated with N₂ andcovered with an oil layer. Into the compartment adjacent to Ox-I yellowleucoindigo carmine was introduced. Then the catalyst system wasirradiated from both sides with a 500 Watt halogen lamp through 420 nmcut-off filters. After acidifying the compartment adjacent to Red-IIwith sulfuric acid to a pH value of about 5 at ambient temperature,hydrogen and oxygen developed as evident by the change in color of theyellow leucoindigo carmine to the blue indigo carmine (a sensitiveindicator for oxygen).

7.3.a Combining Ox-I and Red-III

Both of the catalyst units (slide Ox-I and slide Red-III) produced inExample 1 and Example 6, respectively, comprising theoxidation-promoting first photoactive material RuS₄ and thereduction-promoting second photoactive material platinized SrTiO₃codoped with 2.5% antimony and 2.0% chromium, respectively, are bondedtogether by their non-coated faces and the coated surfaces of the twounits are conductively interconnected by a copper conductive adhesivetape (made by PGO Präzisions Glas & Optik GmbH, Im Langen Busch 14,D-58640 Iserlohn, Germany).

7.3.b Irradiating the Catalyst System (Ox-I)-(Red-III)

A catalyst system as made in Section 7.3.a was adhered to a Nafion®membrane and a window was cut into the membrane so that both sides ofthe system were exposed. The membrane comprising the catalyst system wasinserted into a photoreactor so that two compartments were formed. Bothcompartments were filled in deoxygenized water saturated with N₂ andcovered with an oil layer. Into the compartment adjacent to Ox-I yellowleucoindigo carmine was introduced. Then the catalyst system wasirradiated from both sides with a 500 Watt halogen lamp through 420 nmcut-off filters. At ambient temperature, hydrogen and oxygen developedas evident by the change in color of the yellow leucoindigo carmine tothe blue indigo carmine (a sensitive indicator for oxygen).

7.4.a Combining Ox-II and Red-III

Both of the catalyst units (slide Ox-II and slide Red-III) produced inExample 2 and Example 6, respectively, comprising theoxidation-promoting first photoactive material WO₃ (platinized as wellas non-platinized) and the reduction-promoting second photoactivematerial platinized SrTiO₃ codoped with 2.5% antimony and 2.0% chromium,respectively, are bonded together by their non-coated faces and thecoated surfaces of the two units are conductively interconnected by acopper conductive adhesive tape (made by PGO Präzisions Glas & OptikGmbH, Im Langen Busch 14, D-58640 Iserlohn, Germany).

7.4.b Irradiating the Catalyst System (Ox-II)-(Red-III)

A catalyst system as made in Section 7.4.a was adhered to a Nafion®membrane and a window was cut into the membrane so that both sides ofthe system were exposed. The membrane comprising the catalyst system wasinserted into a photoreactor so that two compartments were formed. Bothcompartments were filled in deoxygenized water saturated with N₂ andcovered with an oil layer. Into the compartment adjacent to Ox-I yellowleucoindigo carmine was introduced. Then the catalyst system wasirradiated from both sides with a 500 Watt halogen lamp through 420 nmcut-off filters. At ambient temperature, hydrogen and oxygen developedas evident by the change in color of the yellow leucoindigo carmine tothe blue indigo carmine (a sensitive indicator for oxygen).

7.5.a Combining Ox-III and Red-III

Both of the catalyst units (slide Ox-III and slide Red-III) produced inExample 3 and Example 6, respectively, comprising theoxidation-promoting first photoactive material Mn₄O₄(phenyl₂PO₂) and thereduction-promoting second photoactive material platinized SrTiO₃codoped with 2.5% antimony and 2.0% chromium, respectively, are bondedtogether by their non-coated faces and the coated surfaces of the twounits are conductively interconnected by a copper conductive adhesivetape (made by PGO Präzisions Glas & Optik GmbH, Im Langen Busch 14,D-58640 Iserlohn, Germany).

7.5.b Irradiating the Catalyst System (Ox-III)-(Red-III)

A catalyst system as made in Section 7.5.a was adhered to a Nafion®membrane and a window was cut into the membrane so that both sides ofthe system were exposed. The membrane comprising the catalyst system wasinserted into a photoreactor so that two compartments were formed. Bothcompartments were filled in deoxygenized water saturated with N₂ andcovered with an oil layer. Into the compartment adjacent to Ox-I yellowleucoindigo carmine was introduced. Then the catalyst system wasirradiated from both sides with a 500 Watt halogen lamp through 420 nmcut-off filters. At ambient temperature, hydrogen and oxygen developedas evident by the change in color of the yellow leucoindigo carmine tothe blue indigo carmine (a sensitive indicator for oxygen).

The entire disclosure of all documents cited in the present application,such as e.g. journal articles, books as well as patents and patentapplications, is herein incorporated by reference.

What is claimed is:
 1. A monolithic catalyst system for the cleavage ofwater into hydrogen and oxygen with the aid of light, wherein thecatalyst system comprises (a) a first photoactive material which, whenirradiated with light having a wavelength≧420 nm, is capable ofgenerating oxygen and protons from water either by itself or togetherwith at least one of an auxiliary material and an auxiliary catalyst,and (b) a second photoactive material which, when irradiated with lighthaving a wavelength≧420 nm, is capable of reducing protons in water tohydrogen either by itself or together with at least one of an auxiliarymaterial and an auxiliary catalyst, (a) and (b) being in electricalcontact via (c) one or more electron-conducting materials.
 2. Thecatalyst system of claim 1, wherein the wavelength is ≧450 nm.
 3. Thecatalyst system of claim 1, wherein the system comprises one or moreauxiliary materials and/or catalysts which are in association with atleast one of (a) and (b).
 4. The catalyst system of claim 1, wherein (c)comprises at least one of a metal, a metal alloy, and an oxidicelectron-conducting material.
 5. The catalyst system of claim 1, whereinthe system comprises a substrate.
 6. The monolithic catalyst system ofclaim 1, wherein (a) comprises one or more materials selected fromoptionally doped oxide- and/or sulphide-containing materials, complexesand clusters comprising at least one of a noble metal and a transitionmetal, and photoactive polymeric materials.
 7. The catalyst system ofclaim 1, wherein (b) comprises one or more materials selected frommetal-containing complexes, organic compounds having an extended πsystem, oxide and/or oxynitride containing materials, phosphide,arsenide, antimonide, sulphide and/or selenide containing materials, andphotosemiconducting polymers.
 8. The catalyst system of claim 1, whereinat least one of (a) and (b) is bound to (c) by a functional group. 9.The catalyst system of claim 1, wherein the system comprises a planemultilayer structure, one side of the structure comprising (a) and theother side of the structure comprising (b), or one side of the structurecomprising both (a) and (b).
 10. A method of generating oxygen andhydrogen from water with the aid of light and a catalyst system, whereinthe method comprises contacting a monolithic catalyst system comprising(a) a first photoactive material which, when irradiated with lighthaving a wavelength≧420 nm, is capable of generating oxygen and protonsfrom water either by itself or together with at least one of anauxiliary material and an auxiliary catalyst, and (b) a secondphotoactive material which, when irradiated with light having awavelength≧420 nm, is capable of reducing protons in water to hydrogeneither by itself or together with at least one of an auxiliary materialand an auxiliary catalyst, (a) and (b) being in electrical contact via(c) one or more electron-conducting materials, with water or an aqueousfluid or solution at a first location comprising the first photoactivematerial or at least one of an auxiliary material and an auxiliarycatalyst associated therewith, and at a second location comprising thesecond photoactive material or at least one of an auxiliary material andan auxiliary catalyst associated therewith, and thereafter irradiatingwith light the water or aqueous fluid or solution in contact with thefirst location and the water or aqueous fluid or solution in contactwith the second location, the first and second locations being incontact with each other such that protons can migrate from the firstlocation to the second location.
 11. The method of claim 10, wherein thelight comprises sunlight.
 12. The method of claim 10, wherein the firstlocation and the second location are separated from each other by amembrane that is permeable only for protons and water.
 13. The method ofclaim 10, wherein the first location is directly irradiated with light.14. The method of claim 10, wherein the second location is directlyirradiated with light.
 15. The method of claim 10, wherein the firstlocation and the second location location are directly irradiated withlight.
 16. The method of claim 10, wherein at least one of oxygen andhydrogen are collected intermittently or continuously.